1. Field of the Invention
The present invention relates to the field of micro-injecting devices, and more particularly to the cohesion of a protective layer and a heating chamber barrier layer in a micro-injection device.
2. Description of the Related Art
Generally, a micro-injecting device refers to a device which is designed to provide printing paper, a human body or motor vehicles with a predetermined amount of liquid, for example, ink, injection liquid or petroleum using the method in which a predetermined amount of electric or thermal energy is applied to the above-mentioned liquid, yielding a volumetric transformation of the liquid. This method allows the application of a small quantity of a liquid to a specific object.
The inkjet printer is a form of micro-injecting device which differs from conventional dot printers in the capability of performing print jobs in various colors by using cartridges. Additional advantages of inkjet printers over dot printers are lower noise and enhanced quality of printing. For these reasons, use of inkjet printers is increasing.
An ink-jet printer includes a printhead with a plurality of nozzles having a minute diameter. The printhead performs a printing performance by bubbling and expanding an ink and spraying the ink on a printing paper.
Examples of the construction and operation of several ink jet print heads of the conventional art are seen in the following U.S. patents. U.S. Pat. No. 4,490,728, to Vaught et al., entitled Thermal Ink Jet Printer, describes a basic print head. U.S. Pat. No. 4,809,428, to Aden et al., entitled Thin Film Device For An Ink Jet Printhead and Process For Manufacturing Same and U.S. Pat. No. 5,140,345, to Komuro, entitled Method Of Manufacturing a Substrate For A Liquid Jet Recording Head And Substrate Manufactured By The Method, describe manufacturing methods for ink-jet printheads. U.S. Pat. No. 5,274,400, to Johnson et al., entitled Ink Path Geometry For High Temperature Operation Of Ink-Jet Printheads, describes altering the dimensions of the ink-jet feed channel to provide fluidic drag. U.S. Pat. No. 5,420,627, to Keefe et al, entitled Ink Jet Printhead, shows a particular printhead design.
Typically, such a conventional ink-jet printhead utilizes a high temperature generated by a heating resistor layer to spray an ink outside. However, when the high temperature influences upon an ink for a great amount of time, thermal changes may occur in the components of the ink, which results in a reduced durability of a device containing the ink, for example, an ink chamber.
Recently, in order to overcome such a problem of reduced durability, a new type of micro-injection device has been developed in which a plate-shaped membrane is applied between the heating resistor layer and the ink chamber. The dynamic deformation of the membrane is induced by the vapor pressure of a working liquid filled in a heating chamber so that the ink in the ink chamber can be efficiently sprayed out. In this case, the membrane inserted between the ink chamber and the heating resistor layer prevents the heating resistor layer from being brought in direct contact with the ink, thereby minimizing any thermal changes of the ink. An example of this type of printhead is seen in U.S. Pat. No. 4,480,259, to Kruger et al., entitled Ink Jet Printer With Bubble Driven Flexible Membrane.
Accordingly, the ink-jet printhead including a membrane is formed by overlaying the heating resistor layer, the heating chamber, the membrane, the ink chamber and the nozzles on a substrate of, for example, a silicon material. In such an inkjet printhead, the heating resistor layer formed on the substrate and defined by a heating chamber barrier layer is supplied with electric energy from outside by contacting with an electrode layer. However, since the electrode layer is in contact with the substrate as well as the heating resistor layer, there is a problem that the electric energy flowing through the electrode layer is leaked out through the substrate.
In the prior art, to prevent the leakage of the electric energy through the substrate, a protective layer of, for example, SiO.sub.2 is formed on the substrate so that the electrode layer can be insulated from the substrate and thereby the electric energy flowing through the electrode layer cannot leak out through the substrate. At this time, the protective layer is brought in contact with all of the electrode layer, the heating resistor layer, the heating chamber barrier layer.
Typically, the heating chamber barrier layer formed on the protective layer is formed of a polyimide material because of the chemical stability of this material with respect to the working liquid filled in the heating chamber, and the protective layer is formed of SiO.sub.2, which is quite different from the material of the heating chamber barrier layer, because of the insulation requirement between the electrode layer and the substrate. However, in this case, there is a reduced cohesion of the heating chamber barrier layer and the protective layer due to the difference in the materials.
Due to the reduced cohesion of the heating chamber barrier layer and the protective layer, a gap is formed between the heating chamber barrier layer and the protective layer. As a result, the working liquid filling the heating chamber flows through the gap and leaks to the protective layer.
In this case, the leaking working liquid erodes the protective layer and the protective layer is damaged. As a result, the insulation capability of the protective layer is markedly reduced. As temperature and humidity are frequently changed, the cohesion of the heating chamber barrier layer and the protective layer is enormously reduced.
As mentioned above, when a firm cohesion of the heating chamber barrier layer and the protective layer is not present, the heating chamber barrier layer cannot be firmly formed on the protective layer. As a result, the finally formed heating chamber barrier layer has an inequality in the thickness.
At this time, if a thermal treatment for improving the durability of the heating chamber barrier layer is further added, the cohesion of the protective layer and the heating chamber barrier layer is further greatly reduced. As a result, the general printing quality of the printhead is markedly reduced.